The problem of preventing halation in photographic elements is well known and various means have been designed to overcome this phenomenon. The photographic materials must be protected against the halation phenomenon which is caused by a light reflected from the base of the photographic material to the light sensitive layers. Said halation defect can be removed by coating a light-absorbing layer (generally called antihalo layer) on one of the surfaces of the support, usually the rear surface, so that rays which would otherwise be reflected onto the sensitive layer are thereby absorbed. Such antihalo layers generally consist of a dye or a pigment dispersed in a natural or synthetic binding resin.
There are basically two alternatives in the formulation of such antihalo layers: 1) the "decolorizable" type wherein dyes or the pigments included in the antihalo layer are decolorized during the processing, and 2) the "removable" type, wherein the binding resin is soluble during processing. In the removable type of antihalo layer binding resins, which must be soluble in the alkaine water solutions, and carbon black or colloidal carbon dispersed in such resin are used.
Colloidal carbon dispersions useful in antihalation backings have been described in U.S. Pat. No. 2,271,234 where an alkali soluble cellulose ester, in particular cellulose acetate phthalate, is used as binders for antihalation layers. Using a carrier of that type, as also described in U.S. Pat. No. 2,327,828, the antihalation layer is removable upon treatment in alkaline photographic developers. U.S. Pat. No. 3,392,022 discloses a photographic film support provided with a layer comprising a cellulose ester containing acetate radicals and radicals selected from the group consisting of hexahydrophthalyl and tetrahydrophthalyl and antihalation material.
In some recent development systems for black-and-white and color photography, the film is submerged in a preliminary imbibing (non-removing) bath, which is an alkaline solution of a high salt content. This solution makes the antihalo layer soluble (for example, a prebath or a backing removal solution); then after leaving the alkaline solution, the film is washed with water to remove the antihalo layer. It may be desirable furthermore to coat a magnetic track on the side of the support freed from said antihalo layer.
As a rule, the antihalo layer should not dissolve in such imbibing bath, which merely must salify the acid groups of the binding resin to give them the solubility in water. To prevent the contamination of the imbibing bath, the resin of the antihalo layer has to remain compact and must not dissolve in it, even if it is salified in said imbibing bath. Non-substantial removal of the antihalo layer in the imbibing bath is in fact necessary to avoid possible scratches on the film. During a following washing step with water, prior to entering the first developing bath, the antihalo layer must be easily and totally removed. Sometimes, in such washing step with water, jets of water under pressure are used to remove the whole layer perfectly.
It is very difficult to formulate antihalo layers which can be totally removed by means of mere water jets. If resins easily soluble in the alkaline water solutions are used to allow the total removal by means of mere water jets, the integrity of the anti-halo layer results to be too low to withstand the imbibing bath, such that it tends to contaminate the imbibing bath itself. If, on the contrary, a resin with a lower solubility in the alkaline water solution is used to have an integrity in the imbibing bath such as not to contaminate it, the removability of the antihalo layer by means of mere water jets is not totally assured and a thin veil of antihalo film may remain on the supporting base under the form of fog. Usually in the photographic process under consideration, after or during the washing steps with water jets, the photographic material dipped into current water is put in contact with rotating rollers covered with a soft material or with brushes provided with bristles, to remove the residual antihalo layer. Less than perfect functioning of the mechanical action of the rollers or brushes, due to wear or defects of the same, often causes an incomplete removal of the antihalo layer.
A material having an antihalo layer which retains its integrity in the imbibing bath and is easily removable in a subsequent washing step is described in U.S. Pat. No. 4,039,333; this layer includes a mixture of resin substantially consisting of a terpolymer containing from 10 to 25% of alkylacrylate units, from 52 to 65% of alkylmethacrylate units and from 18 to 30% acrylic or methacrylic acid, and a resin soluble in aqueous alkali solution. Such a layer, however, is not completely removable with water jets without the mechanical action exerted by the contact with the rotating rollers or brushes.
U.S. Pat. No. 4,262,088 describes a formulation for carbon black antihalation layer which comprises a mixture of two copolymers of low molecular weight alkylmethacrylate and acrylic or methacrylic acid. A resinous viscosity control agent soluble in alkaline solution, such as, for example, hydroxypropylmethyl cellulose phthalate, is also added to said composition.
Japanese Patent Application No. 61-094045 describes an antihalation layer which includes cellulose mixture ester having hexahydrophtharyl group or tetrahydrophtharyl group and hydroxyalkylalkyl cellulose acetate phthalate. Antihalation layer has improved humidity resistivity and is easily removable.
U.S. Pat. No. 3,870,702 discloses a coating composition consisting essentially of a solution of an organic solvent and at least one monoester selected from the group consisting of a tetrahydrophthalic acid monoester and hexahydrophthalic acid monoester of a cellulose ether.
U.S. Pat. No. 4,226,981 describes a method for preparing mixed ester selected from the class consisting of alkylcellulose, hydroxyalkylcellulose and hydroxyalkylalkyl cellulose esterified with acidic succinyl groups and acyl groups. The method for providing a halation preventing layer on the surface of a photographic film which comprises applying said solution of mixed esters is also described therein.
PCT application No. 97/38016 discloses a modified cellulose ester having formula:{(C.sub.6 H.sub.7 O.sub.2)(OR).sub.x (OR').sub.y (OH).sub.3-x-y } and characterized in that R is hydrophobic, R' is hydrophilic, relative to each other, but R' is not phthalyl or trimellityl and x, y and n are selected so that the modified cellulose ester will dissolve in aqueous solutions having a pH at or above 6.5, and not below.
Alkaline soluble cellulose derivatives are known to be used also as protective coatings on pharmaceutical solid dosage forms, particularly enterosoluble coatings. Properties generally required of enteric coating agents for medicament are insolubility in the stomach, solubility in the intestines and high moisture permeability resistance and stability. For such purpose, cellulose acetate phthalate has been widely employed, but they do not possess moisture permeability resistance and stability, so that during storage, it is liable to be hydrolized by the moisture contained in the air, thereby isolating acetic acid and greatly reducing the commercial value of the medicament. Other well known cellulose ethers and ether derivatives have not good enough solubilities in ordinary organic solvents, such as acetone, so that their coating methods are limited. Furthermore, they have poor moisture permeability resistance, which are apt to cause the medicaments treated with them to be degenerated while they are stored.
The cellulose derivatives described in the art do not have stability against hydrolysis when subjected to conditioning at high relative humidity and temperature.